A dramatic increase in the information capacity of an optical fiber can be achieved by the simultaneous transmission of optical signals over the same fiber from many different light sources having properly spaced peak emission wavelengths. By operating each source at a different peak wavelength, the integrity of the independent messages from each source is maintained for subsequent conversion to electric signals at the receiving end. This is the basis of wavelength division multiplexing (WDM).
Wavelength switched optical networks potentially offer high capacity networking at lower cost than current electronically switched networks. The rare-element doped fiber amplifiers such, for example, as erbium doped fiber amplifiers (EDFA's) in the nodes and repeaters of such networks will each be traversed by multiple signal channels following diverse routes. In optical amplifiers such as EDFA's, amplified spontaneous emission (ASE) is the major source of noise. ASE originates from the spontaneous emission of incoherent light over the full gain bandwidth of the amplifier. This is the random noise contribution of the amplifier. In the wavelength domain, gain saturation induced by a data channel operating at .lambda..sub.1 produces a level change in another data channel at wavelength .lambda..sub.2. If the optical powers in the transmission fibers are too high, optical nonlinearities, such as Stimulated Brillouin Scattering, can occur and degrade the signals.
In optically amplified systems, the above described noise sources present two limitations on the amplifier operating range. At low input signal levels the amplifier random noise contribution, ASE, causes bit errors (signal-spontaneous beat noise) while at large input signal levels, nonlinearities in the transmission medium can also add noise which degrades performance.
Although automatic gain control circuit (AGC) employing laser feedback at the detection end can compensate for fluctuations in the data stream that occur on the order of 1 ms, significant crosstalk between channels can occur for multichannel applications such as wavelength division multiplexing (WDM), especially when channel(s) is/are dropped or added to the system. The number of channels traversing an EDFA may, in fact, change suddenly as a result of a network reconfiguration or a fault that interrupts some of the channels. To prevent performance penalties in a high capacity network, the power excursions experienced by the surviving channels--those which share an amplifier with the channels directly affected by the fault or reconfiguration--should be limited when channels are added and when channels are dropped.
The gain medium in an EDFA has a comparatively long excited state lifetime or relaxation time, and for this reason is generally regarded as allowing for a larger saturation energy and hence, as exhibiting virtually no modulation of the saturation level at high speed data pulses (pulse period&lt;1 ns). In fact, it has been reported that transient effects of gain saturation and recovery in an individual amplifier typically occur on a 100 .mu.s-1 ms time scale. Desurvire et al, Erbium Doped Fiber Amplifiers, 1994, p. 412. The inventors herein have, however, observed gain dynamics in a chain of EDFA's almost two orders of magnitude faster than this and, for large scale wavelength routed networks, gain dynamics three orders of magnitude faster may be predicted. These fast transients in chains of amplifiers may ultimately constrain the design or extent of multi-wavelength optical networks. Accordingly, a need is recognized for dynamic gain control fast enough to ensure reliable service in high capacity networks employing significant numbers of optical amplifiers in the communication paths thereof.